1. Field of the Invention
The invention relates to techniques for forming an AlGaN crystal layer on an AlN template substrate.
2. Description of the Background Art
There is a known technique for fabricating a template substrate (epitaxial substrate) that may be used as a base substrate in the process of epitaxially forming a Group III nitride crystal layer (see for example Japanese Patent Application Laid-Open (JP-A) No. 2006-332570).
It is also known that a diode structure-type light-emitting device having an emission wavelength in the ultraviolet region is obtained by forming a light-emitting layer or an n-type or p-type conductive layer of high-Al-content AlxGa1−x N (x≧0.4) (see for example “III-Nitride UV Devices,” M. Asif Khan, M. Shatalov, H. P. Maruska, H. M. Wang, and E. Kuokstis, Jpn. J. Appl. Phys., Vol. 44, No. 10, 2005, pp. 7191-7206).
In order to prepare a light-emitting device having an emission wavelength in the deep ultraviolet region as disclosed in the article of Khan et al., it is necessary to form laminations of a plurality of crystal layers made of high-Al-content AlxGa1−x N (x≧0.4) on a single crystal substrate of sapphire or SiC. In order to obtain the light-emitting device with good characteristics, the lamination should be formed in a good manner, and therefore each crystal layer is required to have a good surface flatness.
JP-A No. 2006-332570 discloses that a template substrate having a crystal layer with improved surface flatness can be obtained by a process that includes forming a crystal layer made of Group III nitride such as an AlN as a surface layer on a single crystal base material and then performing a heat treatment at a temperature higher than the temperature at which the crystal layer is formed. However, the inventors have found that when a high-Al-content AlGaN layer as disclosed in the article of Khan et al. is formed using a template substrate having the AlN surface layer, the resulting AlGaN layer does not always have a good surface flatness, although it has a good crystallinity.
In addition, the article of Khan et al. also discloses an aspect that in order to prevent cracking, a multilayered buffer layer is formed before the high-Al-content AlGaN layer is formed. In this case, however, crystal layers having a good surface flatness also cannot be obtained.